Plugin for Roundcube webmail for two-factor authentication, allowing
the choice (per-account) of requiring YubiKey authentication in addition
to standard password authentication.

Note: this normally connects to a YubiKey API server over https.
If your web server runs in a chroot jail, you must copy /etc/ssl/cert.pem
(or another suitable certificate bundle) to /var/www/etc/ssl/cert.pem.
